The Ghost of Fireballs Past

The Ghost of Fireballs Past

December 21, 1998: A little more
than a month ago the Leonid meteors put on a dazzling show of
shooting stars and fireballs for sky-watchers around the world.
While most meteor enthusiasts were outside watching the show,
a few stayed inside and tuned into the shower on their radios.
What they heard may surprise you.

Meteoroids like the Leonids travel at tremendous velocities.
When they strike Earth's atmosphere at speeds of 100,000 mph
or more they ionize the air in their path. These luminous ionized
trails, like the one pictured below, are visually striking. But
that's not all -- they also reflect radio waves. During a shower
like the Leonids, radio signals from TV stations, RADAR facilities,
and AM/FM transmitters are constantly bouncing off meteor trails.
The echoes can be heard around the world.

Left: This photo of a Leonid was
captured by Steve Dunn (copyright 1998, all rights reserved)
near Cape Canaveral, Florida around 0600 EST on Nov. 17, 1998.
The colorful tail consists of ionized air that can reflect radio
waves from TV, radar, and AM/FM radio transmitters.

One of the most powerful transmitters on Earth is the Navy Space
Surveillance Radar (NAVSPASUR for short). Located in Kickapoo,
TX, NAVSPASUR transmits 800 kW of continuous-wave (CW) radio
power into an east-west oriented fan beam at 216.98 MHz. The
radar's primary mission is to track satellites and space debris
for the US Space Command. It can detect objects as small as 10
cm orbiting 15,000 km above the earth's surface. Although meteoroids
measuring a mm or less in size are generally too small to be
detected, their ion trails produce strong echoes that can be
heard on simple ham radio equipment.

Amateur radio enthusiast
Dr. Steven Bienvenu monitored NAVSPASUR during the November Leonid
meteor shower and also during this month's Geminid shower. He
recorded strong meteor echoes on both occasions. The graphic
pictured left is a 25-second snippet of his receiver's audio
output during the Leonid meteor shower. The strongest echoes
are reflections from Leonid meteor trails; the weaker and briefer
"blips" are satellite echoes.

The WAV-format audio files linked below
contain recordings of NAVSPASUR radar echoes from Leonid and
Geminid meteors, and from the space shuttle Endeavour during
the recent ISS assembly mission. Each file was extracted from
an audio recording provided by Dr. Steven Bienvenu, M.D.

Leonid meteor echo: This 25s audio file begins
with 3 brief satellite echoes in rapid succession followed by
a 15+ second Leonid meteor radar reflection. The data were obtained
on November 17, 1998 during the Leonid meteor shower.208 kB WAV

Geminid meteor echo: This 10s audio file includes
a radar echo from a Geminid meteor lasting approximately 3 seconds.
The recording was made during the Geminid meteor shower on December
14, 1998.218 kB WAV

Satellites move through the NAVSPASUR beam rapidly, usually in
less than a second. As a result they sound like the "blips"
and "pings" that are stereotypical of radars and sonars.
(In the three audio recordings above, only the Leonids WAV file
contains echoes of fast-moving satellites.)

Meteor trails are longer-lasting. They usually persist for a
few seconds, but in extreme cases they can last for 10 or more
minutes. They sound less like blips, and more like long-winded
warbling whistles.
Meteors, satellites, and spacecraft have even more distinctive
signatures in the dynamic spectra of NAVSPASUR radar returns.
A full discussion of radar spectra is beyond the scope of this
article. More information, plus colorful echo spectra of meteors
and the space shuttle Endeavour may be found here.
Reports from Dr. Bienvenu and other amateur radio operators indicate
that the November 1998 Leonids were far better radio meteors
than December's Geminids. The Leonids were more numerous and
their reflective tails lasted longer. One amateur radio operator
reportedly exchanged calls with 14 other hams during a 9-minute
long Leonid "burn." In contrast, the 1998 Geminids
featured very few "long burners."

Most meteor echo enthusiasts listen for reflections of distant
radio transmitters at frequencies between 40 MHz and 100 MHz.
These are the best bands for such work because weakly ionized
meteor trails reflect signals most efficiently at lower frequencies,
below 100 MHz. Although 216 MHz would be considered by most to
be a poor frequency for meteor observations, because it is too
high for efficient reflections, the tremendous power of the Naval
Space Surveillance radar transmitter more than compensates for
its less-than-optimum observing frequency.

Amateurs in much of North America and in the Caribbean should
be able to detect NAVSPASUR echoes from bright meteors and the
larger space satellites using relatively modest ham radio equipment.
For example, Dr. Bienvenu's NAVSPASUR observing setup was relatively
simple. He used a ICOM R7000 radio receiver in USB mode tuned
to 216.980 MHz, with a slight frequency offset to center the
NAVSPASUR signal in the narrow 2.5 kHz receiver band pass. The
antenna was a common TV/FM Yagi-style aerial. During the Leonid
and Geminid meteor showers he pointed the antenna toward azimuth
275 degrees, the direction of the NAVSPASUR transmitter as seen
from his location in Louisiana. Meteor trails are formed just
80 km above Earth's surface, so he pointed the Yagi just 8 degrees
above the horizon.

He recorded the audio output of his receiver at a rate of 11,000
samples per second using the sound card in his 120 MHz Pentium
PC.

This approach to meteor observing could prove fruitful for showers
that are difficult to observe visually, including daytime meteor
showers, and showers that take place during the full moon. The
upcoming Quadrantid meteor shower is a perfect example. As many
as 120 meteors per hour are expected during the shower's peak
on January 3, 1999, but the full moon will make observations
of all but the brighter fireballs difficult. It may be a good
year to forgo the cold outdoors and listen to the Quadrantids
on the radio instead.
Web Links

Dynamic
Spectra of Meteor Radar Echoes
(or, How to distinguish meteor and satellite
echoes in one easy lesson)

One way to distinguish between satellite and meteor echoes is
by simply listening. Satellites sound like "blips"
or "pings". Meteors are longer-lasting warbling whistles.

A more clear-cut approach is to plot the power spectrum of the
radar echo as a function of time. In a "dynamic spectrum"
the signature of a meteor is clearly different from that of a
satellite or spacecraft.

Satellites race through the NAVSPASUR radar beam at velocities
between 1 and 10 km/s, depending on the details of their orbit.
Not only is the reflection brief, it is also Doppler shifted
as the orientation of the satellite's velocity vector changes
with respect to the radar transmitter. In a dynamic spectrum
a satellite reflection appears as a slanted, nearly vertical
line (see below).

Dynamic spectrum showing
Leonid meteor and satellite radar echoes
This dynamic spectrum
spans the first 11 seconds of the Leonid
meteor echo WAV file discussed in the body
of this article. Power is displayed in false color. Blue
is low power; red is high.

Meteoroids move at much higher velocities than low Earth orbit
satellites, so the Doppler shift from a meteoroid echo would
be even greater than that of a satellite. However, it is not
the fast-moving meteoroid that causes the radar echo. The radio
signal bounces off the ionized air left behind when the meteoroid
disintegrates in the atmosphere. The velocity of the ionized
trail is low, typically no more than 0.02 km/s, compared to 30
- 70 km/s for the meteoroid. As a result there is no observable
Doppler shift in the meteor echo. The meteor echo appears in
the dynamic spectrum as a nearly horizontal line exhibiting no
drift in frequency.

Dynamic spectrum of
a radar echo from STS-88

On December 14, 1998 near the peak
of the Geminid meteor shower, Dr. Steven Bienvenu obtained an
audio recording of NAVSPASUR echoes from the space shuttle Endeavour.
The echo describes a slanted line in the frequency-time domain
that is characteristic of the doppler shifted echoes from satellites
and spacecraft, but not from meteor trails. Power is displayed
in false color. Blue is low power; red is high.